[5cienceboy]

Using molecular orbital theory explain why He2 is unlikely to exist and why He2+ could be stable?

Is it just that because the electrons fill up the first level, they wont bond? But when you remove an electron its not full anymore? im not great on mo theory in general but this ones stumped me. any help appreciated.

[cpncoop]

Look at the He atom - Its electron configuration is 1s2.  For MO theory, you need to combine the molecular orbitals of the two atoms in question.  In this case, you combine 2 1s orbitals.  As you have alluded to, you have a bonding orbital that is of a lower energy than the 1s orbital by itself, and an antibonding orbital of higher energy.  With He 2, you end up with 4 total electrons, 2 of which would be in a bonding orbital, and 2 of which would be in an anti-bonding orbital (don't forget to fill the orbitals according to the Pauli Exclusion Principle!).  To calculate the bonding character of the interaction, subtact the bonding electrons from the nonbonding electrons.  For He2, that equals 0, so no bonding can take place, hence it is unstable.  For He2+, you only have 3 electrons, so you have 2 bonding electrons - 1 non-bonding electron, giving the interaction a slight bonding character (hence it may be stable).  I hope this helps,